The invention relates generally to light emitting devices and more particularly to a light emitting diode with improved brightness and reliability and to a method of fabricating the light emitting diode.
Traditional lighting sources such as fluorescent lights, incan-descent lights, and neon lights have a number of disadvantages relative to light emitting diodes (LEDs). These disadvantages include their large sizes, the lack of durability due to the use of fragile filaments, short operating lifetimes, and high operating voltages. In contrast, LEDs are small in size, durable and require low operating voltages. Furthermore, LEDs have much longer operating lifetimes. A typical LED has an operating life of 10,000 hours or more, as compared to a halogen lamp which has a mean operating life of 500 to 4,000 hours. Furthermore, unlike the traditional lighting sources which fail by filament breakage, an LED fails by a gradual reduction in light output. Therefore, many lighting applications could benefit from the advantages of LEDs. However, to effectively compete with the traditional lighting sources, LEDs must be bright and must maintain their brightness over their expected operating lifetime, i.e., they must be reliable.
Many attempts have been made to improve the brightness of LEDs through various design changes. For example, improvements in LED brightness have been achieved by the use of more than one light emitting layer, where these light emitting layers are commonly known as active layers. These LEDs are referred to as either multi-well (MW) LEDs if the layer thickness values are greater than 100 Angstroms, or they are referred to as multiple quantum well (MQW) LEDs if the layer thickness values are less than approximately 100 Angstroms. The distinction between these two different types of LEDs is whether the layers are thin enough for quantum confinement effects to become important, i.e., for discrete or quantized energy states to occur in the active layers. (This generally occurs for well thickness values less than approximately 100 Angstroms and depends on the energy band structure of the materials in question.) In contrast, LEDs having a single active layer will be referred to as either double heterostructure (DH) LEDs, or as single quantum well (SQW) LEDs, again depending on whether the individual active layer thickness values are greater than or less than approximately 100 Angstroms, respectively. Further improvements have been attempted by adjusting the number and/or thickness of the active layers. For example, U.S. Pat. No. 5,410,159 to Sugawara et al. describes an MQW LED having eight to nineteen active layers, preferably ten to nineteen, in which the thickness of the active layers is 10 to 100 Angstroms, and more typically 50 Angstroms. The device described by Sugawara et al. is thus restricted to the quantum regime.
A conventional MQW LED is schematically illustrated in FIG. 1. (Note that this LED in FIG. 1 could also represent an MW LED.) The LED 10 includes a substrate 12 of a first conductivity type, a lower confining layer 14 of the first conductivity type, the MQW active region 16 which may be of the first conductivity type, may be undoped, or may be of a second conductivity type, an upper confining layer 18 of the second conductivity type, and an optional window layer 20 of the second conductivity type. The MQW active region includes two or more active layers 22 that are separated from each other by one or more barrier layers 24. Although the MQW active region is shown to include four active layers, the number of active layers included in the active region can be much greater.
In the most common configuration, the first conductivity type is ntype and the second conductivity type is p-type. Since this is the most common LED configuration, such a configuration will be used here as an example. In this configuration, the n-type lower confining layer 14 of the LED 10 in FIG. 1 is electrically connected to an n-type ohmic contact 26 via the substrate 12, and the p-type upper confining layer 18 or the optional window layer 20 is electrically connected to a p-type ohmic contact 28. (Note that it is also possible to form an LED where the first conductivity type is p-type and the second conductivity type is n-type. Such an LED may be formed by either growing the LED on a p-type substrate, or bonding or attaching the LED to a p-type substrate or other p-type semiconductor material.)
When a potential is applied to the ohmic contacts 26 and 28, electrons are injected into the MQW active region 16 from the n-type lower confining layer 14 and holes are injected into the MQW active region from the p-type upper confining layer 18. The radiative recombination of electrons and holes within the active layers 22 of the active region generates light. However, when the recombination occurs within the lower confining layer, the upper confining layer, or the barrier layers of the active region, no light is generated. Thus, it is desirable to increase the probability that the electrons and holes recombine within the active layers, as opposed to recombining within some other layers of the LED. The multiple quantum wells formed by the active layers of the LED increase the radiative recombination probability by allowing holes or electrons that did not recombine in one of the active layers a chance to recombine in another active layer. The increase in radiative recombination of electrons and holes within the active layers equates to an increase in the light output of the LED.
With respect to the reliability issue, U.S. Pat. No. 5,909,051 to Stockman et al., which is assigned to the assignee of the present invention, describes a method of doping at least one conductive region adjacent to the active region with impurities to fabricate minority carrier devices, such as an AlGaInP LED, having an increased operating stability, i.e., reliability. In a preferred embodiment, the impurities that are introduced to the conductive region are oxygen dopant atoms. Note that the fabrication method of Stockman et al. asserts that the oxygen dopant atoms should be placed in a region adjacent to the active region.
A concern with conventional fabrication methods and the resulting devices, such as those described in Stockman et al., is that improvement in LED brightness and improvement in LED reliability are often in an inverse relationship. Therefore, an improvement in one parameter often results in a penalty in the other parameter. In particular, the method of Stockman et al. provides improved LED reliability. However, it often also results in a penalty in initial light output. Although the method of Stockman et al. results in a more stable light output, these devices with oxygen doping are thus initially dimmer than devices without oxygen doping.
Although improvements in either brightness (i.e., light output as measured by Iv in units of xcexcCd) or in reliability of LEDs may be achieved by conventional methods, additional improvements that minimize any penalizing effects on the other parameter are desired. Therefore, what is needed is an LED fabrication method and a resulting LED that has been configured to optimally improve both the brightness and the reliability of the LED.
An LED and a method of fabricating the LED utilize controlled oxygen (O) doping to form at least one layer of the LED having an O dopant concentration which is correlated to the dominant emission wavelength of the LED. The O dopant concentration is regulated to be higher when the LED has been configured to have a longer dominant emission wavelength. Since the dominant emission wavelength is dependent on the composition of the active layer(s) of the LED, the O dopant concentration in the layer is related to the composition of the active layer(s). The controlled O doping improves the reliability while minimizing any light output penalty due to the introduction of O dopants.
In an exemplary embodiment, the LED is an AlGaInP LED that includes a substrate, an optional distributed Bragg reflector layer, a lower confining layer, an optional lower set-back layer, an active region, an optional upper set-back layer, an upper confining layer, and an optional window layer. The substrate is made of a semiconductor material, such as GaAs. The lower confining layer is composed of an n-type (AlxGa1xe2x88x92x)yIn1xe2x88x92yP material, where xxe2x89xa70.6 and y=0.5xc2x10.1, while the upper confining layer is composed of a p-type (AlxGa1xe2x88x92x)yIn1xe2x88x92yP material where xxe2x89xa70.6 and y=0.5xc2x10.1. The optional upper set-back layer is formed of an undoped (AlxGa1xe2x88x92x)yIn1xe2x88x92yP material where xxe2x89xa70.6 and y=0.5xc2x10.1. The optional upper set-back layer may be used to help control the diffusion of p-type dopants from the upper confining region into the active region during high temperature processing steps. The optional lower set-back layer may also be formed of an undoped or n-type (AlxGa1xe2x88x92x)yIn1xe2x88x92yP material where xxe2x89xa70.6 and y=0.5xc2x10.1. The optional upper and lower set-back layers also generally have an aluminum composition, x, which is less than or equal to the aluminum composition of the upper and lower confining layers, although this is not necessarily the case.
In one embodiment, only the upper confining layer of the LED is doped with a controlled amount of O. Note that the upper set-back layer, which is adjacent to the active region, is not doped with O in this configuration. If the LED is configured to have a dominant emission wavelength less than approximately 595 nm, the O concentration in the upper confining layer of the LED is between about 3xc3x971017 cmxe2x88x923 and 2xc3x971018 cm xe2x88x923. If the LED is configured to have a dominant emission wavelength between approximately 595 nm and approximately 620 nm, the O concentration is between about 4xc3x971017 cmxe2x88x923 and 4xc3x971018 cmxe2x88x923, preferably about 3xc3x971018 cmxe2x88x923. If the LED is configured to have a dominant emission wavelength greater than approximately 620 nm, the O concentration in the upper confining layer is between about 5xc3x971017 cmxe2x88x923 and 5xc3x971018 cmxe2x88x923. However, the O concentration for an LED with a dominant emission wavelength greater than approximately 626 nm can be higher than 5xc3x971018 cmxe2x88x923. The rationale for the different O concentrations is that the light output penalty which is commonly associated with O doping was found to be dependent on the dominant emission wavelength. A shorter dominant emission wavelength equates to a greater light output penalty than a longer dominant emission wavelength. In fact, for a dominant emission wavelength greater than 626 nm, there is an improvement in the light output by the use of O doping. Therefore, by controlling the O concentration in the p-type confining layer, the light output penalty associated with the O doping can be mitigated.
In an alternative embodiment, the p-type set-back layer of the LED is doped with a controlled amount of O, such that the p-type set-back layer contains an oxygen concentration  less than 5xc3x971018 cmxe2x88x923, and preferably  less than 1xc3x971017cmxe2x88x923, where the preferred O concentration in the p-type set-back layer decreases with decreasing dominant emission wavelength. In this case, the upper confining layer, which is not adjacent to the active region, is also O doped.
In addition to the O doping, the upper confining layer of the LED is preferably doped with an increased amount of p-type dopants, such as Mg, Zn, C, or Be. Prior to high temperature thermal processing, the resulting upper confining layer has a p-type dopant concentration between approximately 1xc3x971017 cmxe2x88x923 and 1xc3x971019 cmxe2x88x923. Subsequent high temperature thermal processing then results in p-type dopant diffusion into the active region, resulting in a heavily p-doped active region with a free hole concentration between approximately 1xc3x971017 cmxe2x88x923 and 3xc3x971018 cmxe2x88x923, preferably between 3xc3x971017 cmxe2x88x923 and 6xc3x971017 cmxe2x88x923.
The active region of the LED may include a single light emitting active layer or multiple light emitting active layers. For an LED with multiple light emitting active layers, the total active layer thickness D is defined as the sum of the thicknesses of all the light emitting layers in the active region. As an example, an LED with N light emitting layers, each layer of equal thickness t, the total active layer thickness would be given by Eq. 1 as:
D=Ntxe2x80x83xe2x80x83(1)
For a fixed total active layer thickness, D, it has been found that an increased number of light emitting layers equates to an increase in the light output of the device. An increase in the number of light emitting layers for a fixed total active layer thickness means that the target thickness of the individual light emitting layers is also reduced. The preferred total active layer thickness is in the range from about 500 Angstroms to about 2000 Angstroms for an amber LED (dominant wavelength less than approximately 595 nm) or an orange LED (dominant wavelength between approximately 595 nm and 610 nm), while the preferred total active layer thickness is in the range from about 125 Angstroms to about 1000 Angstroms for a red-orange LED (dominant wavelength between approximately 610 nm and 620 nm) or red LED (dominant wavelength greater than approximately 620 nm). Therefore, if the LED is configured to have a dominant emission wavelength in the amber to orange portion of the light spectrum, the active region includes multiple light emitting layers, ranging in number from approximately eight to approximately sixteen emitting layers, each light emitting layer being approximately 125 Angstroms thick or less. However, if the LED is configured to have a dominant emission wavelength in the red or red-orange portion of the light spectrum, the active region includes multiple light emitting layers, ranging in number from one to approximately eight light emitting layers, each light emitting layer also being approximately 125 Angstroms thick or less. The optimal thickness for these different color LEDs depends on whether the device includes an absorbing substrate (AS) material such as GaAs, or whether the device has been grown on or bonded or otherwise attached to a non-absorbing or transparent substrate (TS) such as GaP, glass, or some other non-absorbing material. The thicker values within the above ranges are preferred for absorbing substrate devices while the thinner values are preferred for transparent substrate devices. Note that the present invention, which is based in part on the concept of reducing the individual active layer thickness, t, and hence increasing the number of active layers, N, will work for layer thicknesses well down into the quantum regime, i.e., for thicknesses down to approximately 10 Angstroms. In such cases, the number of wells, N, may be as high as 50 or more.
The active region of the LED includes N-1 barrier layers that are positioned between two adjacent light emitting layers, where N is the number of light emitting layers included in the active region. It was also found that the light output can be increased by decreasing the thickness of the barrier layers and/or increasing the energy bandgap of the barrier layers. In view of this finding, the thickness of the barrier layers is less than 350 Angstroms, and is preferably 100 Angstroms or less. In addition, the composition of the barrier layers is (AlxGa1xe2x88x92x)yIn1xe2x88x92xP, where x0.6 and y=0.5xc2x10.1. In the preferred embodiment, the composition of the barrier layers is Al05In0.5P, where y=0.5xc2x10.1, and the barrier layers are 100 Angstroms thick or less.
An advantage of the invention is that the unique combination of thin wells, thin barriers, AlInP barrier composition, set-back layer composition, and high doping concentrations of oxygen and acceptor atoms (such as Mg, Zn, Be, or C) allow the light output and reliability to be decoupled to a greater extent than has been observed in prior LED designs. The invention therefore allows simultaneous improvements in both brightness and reliability, as shown in FIGS. 6 and 7 below.